The delivery of medical gases is a commonly accepted medical practice for the treatment of individuals receiving anesthesia during a medical procedure or surgery.
Commonly, a combination of medical gases are supplied to the patient during gas delivery of ventilation. Since the patient requires oxygen to conduct basic bodily functions, the combined medical gases must include a component of oxygen, usually in a concentration of at least the 21% found in ambient air. Higher concentrations of oxygen may be provided to the patient as a form of respiratory therapy whereby oxygen exchange within the lungs is promoted by a higher oxygen concentration. The medical gases with which the oxygen is combined are often referred to as balance gases as they make up the balance of the medical gas delivered to the patient. While the balance gas may be an inert gas such as nitrogen, or a common gas such as air, CO2, or helium, the balance gas may also include medical gases that further provide a therapeutic effect, such as nitrous oxide (N2O), xenon, or a drug aerosol. Therapeutic balance gases may produce the effect of anesthetizing the patient or reducing the patient's airway resistance.
Effective gas delivery allows a clinician the ability to provide a wide range of oxygen-balance gas concentration mixtures such that the proper therapeutic and respiratory support may be provided to the patient. The medical gases are typically provided via a source of compressed medical gas. The compressed medical gas source may be a permanent source built into the medical care facility with medical gas line attachment points located in each of the rooms of the medical facility. Otherwise, the source of compressed medical gas may be a movable cylinder filled with the compressed medical gas. The anesthesia delivery system, made up of a gas delivery system and mechanical ventilator is typically connected to a pressurized source of each of the required medical gases and the gas delivery subsystem of the anesthesia delivery systems must control and facilitate the mixing of the medical gases to achieve the proper concentration of medical gases and flow rates of medical gas as desired and input by the clinician.
There are at least two electronic medical gas mixing systems that are used in mechanical gas delivery systems. A mixing chamber gas mixing system makes use of a separate mixing chamber wherein the medical gases from the pressurized medical gas sources are accumulated at the desired concentration and at a pressure sufficient to deliver the desired flow of medical gas. This system requires proportional flow valves to be associated with each of the pressurized sources of medical gas as well as the mixing chamber such that the proportion of the medical gases going into the mixing chamber may be controlled and the flow rate of the medical gas out of the mixing chamber may also be controlled. While the mixing chamber mixing system provides good control over the concentration and flow rate of the medical gas provided to the patient, the mixing chamber system suffers from a slow response time to any clinician changes to the medical gas concentration. The mixing chamber must be purged of the old medical gas and refilled at the new desired medical gas concentration each time the medical gas concentration is changed.
An alternative mechanical gas delivery mixing system design eliminates the mixing chamber and the flow valves associated with the mixing chamber and simply relies upon the control of proportional flow valves associated with each of the sources of pressurized medical gas. Flows of the pressurized oxygen and balance gas are controlled such that the total desired flow rate is achieved within the mechanical ventilator as well as the desired resulting concentration. While this mixing system provides a fast response time to clinician changes in the desired medical gas concentration, this system suffers from a resolution problem at the high and low ends of the oxygen concentration. This resolution problem is most pronounced at low flow rates but also applies to some degree at high oxygen concentrations throughout the medical gas flow rate spectrum of the gas delivery system. The resolution problem is due to the fact that the concentration of the medical gas is dependent upon the precise control of the flow rate of each medical gas out of the pressurized medical gas sources, which is limited by minimum flow rates required by the flow sensors.